Paper density (also known as paper weight or grammage) is a term used in the printing and paper industries to denote a measure of mass of the paper per unit of area. The term 'density' is not used in its traditional sense of mass per unit volume, rather, it is a measure of the area density.

The most common way of expressing paper density is in grams per square metre (g/m2) or grammage, which is usually written as gsm. The paper density, or grammage, is also used to differentiate paper from board. If a material has a grammage of less than 200gsm we usually refer to this as 'paper', where any material having a grammage of 200gsm and above is usually referred to as 'board'.

Paper thickness, or caliper, is usually expressed in micrometers, or microns. 1 micron is 1/1000mm or 0.001mm. A typical example of a paper thickness would be written as 230micron (0.230mm). Because the true paper density is usually unknown it is impossible to calculate the thickness of any paper based solely on the grammage. It is quite normal that papers from different manufacurers, even though they have the same grammage, will have a different thickness. This can effect how 'bulky' different papers feel when you hold a single sheet in your hands.